U.S. Patent Publication No. 2007-0289759 discloses a hammer drill having a clutch which is disposed in a power transmitting mechanism for transmitting torque of a motor to a tool bit and capable of interrupting torque transmission from the motor to the tool bit when the hammer bit is unintentionally locked during hammer drill operation and thereby preventing reaction torque or excessive torque from acting on a tool body in a direction opposite to the direction of rotation of the tool bit.
In the above-described known technique for preventing reaction torque, the clutch is disposed in the power transmitting mechanism in which the rotation speed of the motor is reduced. Therefore, the size of the clutch is increased in order to allow transmission of high torque. In this point, further improvement is required.